As brent said, the boot / EFI partition should be 1GB minimum. This is shown in the published system requirements
If the installer isn't working to create partitions, use a tool like gparted or KDE Partition Manager (from a live ISO environment) to create the partitions before running the installer. In my experience, that's more reliable. In the installer, you can then use manual partitioning to select the ones you created and assign mount points to them. Good luck.
